The present invention relates to a rock drill bit for percussive drilling comprising a bit head configured to be attached at an end of a drill element of a drilling assembly and having a diameter larger than that of a said drill element, said bit head having at a front end as seen in the intended drilling direction a generally frusto-conical shape defined by a substantially circumferential ring provided with a plurality of gauge buttons distributed along the extension of the ring for engaging material to be crushed, a central flush channel extending through the bit head and having at least one flushing hole opening at said front end, at least one peripheral flushing hole of said at least one flushing hole opening at said front end opens in said substantially circumferential ring, as well as a drilling assembly and a method of percussive rock drilling according to the preambles of the corresponding appended claims.
The invention is not restricted to any type of drilling assembly for use of a said rock drill bit, but the former may be a down-the-hole hammer drill just as well as a top hammer drill.
Furthermore, said rock drill bit may have any conceivable size and has normally a diameter of 30 mm-300 mm. The same absence of limitations applies to the intended percussion frequency and rotational speed of the rock drill bit in operation, although it may be mentioned that these are typically within the ranges 20 Hz-500 Hz and 20-500 revolutions per minute, respectively.
FIG. 1 illustrates a known so-called standard rock drill bit 1 of the type defined in the introduction. The drill bit has a bit head 2 configured to be attached at an end of a drill element, for example in the form of a drill tube or drill rod, of a drilling assembly and having a diameter larger than that of a said drill element. This drill element is not shown in the figure but may be intended to be received in a so-called skirt 3 integral with the bit head and having a smaller diameter than the bit head. Other ways of connecting the drill bit to the drill element are conceivable and known within the art. The bit head has at a front end 4 as seen in the intended drilling direction a generally frusto-conical shape by having a cross-section tapering towards said front by the presence of a substantially circumferential ring 5 provided with a plurality of gauge buttons 6 distributed along the extension of the ring for engaging material to be crushed. These gauge buttons are made of hard material, such as cemented carbide. Front buttons 7 also of hard material are arranged on a front surface 8 for engaging material to be crushed.
Furthermore, the rock drill bit also has a central flush channel extending through the bit head and opening at the front by a flushing hole 9 in the front surface. The flushing medium used will typically be compressed air when drilling is carried out “above earth” and a liquid, such as water, when drilling is carried out with a drilling assembly positioned under ground.
In operation the gauge buttons 6 will engage and break rock close to the walls of a hole in which the rock drill bit with said rod is located and the front buttons 7 will break rock closer to the centre of such a hole by impacts carried out by the rock drill bit in the direction of the arrow A. The drill bit will rotate somewhat, typically about 5°, between each such impact. Cuttings resulting from said rock breaking action of the buttons 6, 7 are to be removed for enabling the buttons to efficiently break new rock during the following impact. This is achieved by the flushing medium emerging from the flushing hole 9 and flushing said cuttings away while escaping rearwardly opposite to the drilling direction A through recesses 10 in the circumference of the bit head and along the skirt 3 and said drill element having a smaller diameter than the bit head and by that than the hole drilled. The cuttings are flushed externally, outside of the drill bit.
The operation efficiency of a rock drill bit of this type is of course an important feature and this may be expressed as the penetration rate of the rock drill bit defined as the length of a hole drilled per time unit (meter/minute). The penetration rate may depend upon the wear of said buttons and the efficiency of the flushing. It is of course an ongoing attempt to increase the penetration rate of a rock drill bit of the type defined in the introduction.
WO 2005/010317 A1 relates to a reverse-circulation down hole face sampling hammer drill, wherein directing of a proportion of the exhaust air up the sample recovery bore tends to reduce the air pressure and volume of flow at the cutting face, reducing bore hole scouring. WO 2004/003334 A1 shows a drill bit provided with a plurality of flush channels. U.S. Pat. No. 6,767,156 B1 discloses a drill bit having a plurality of flush channels. U.S. Pat. No. 4,819,746 A shows a reverse circulation down-the-hole hammer bit. U.S. Pat. No. 3,997,011 A, on which the preamble of claim 1 is based, discloses a rock bit having a plurality of flush channels.